High temperature fluid flow through active décollement at the Nankai subduction zone

Temperature is one of the important parameters to understand complex dynamics, because temperature of the crust is changed by some events such as volcanic activities and a passage of high temperature fluid, which affects physical property, chemical cycle and also microbiosphere. Therefore, information about temperature allow us to understand the dynamics of the active subduction zone.

IODP Site C0023, located at the tip of subduction zone in the Muroto transect of the Nankai Trough, was drilled by IODP Expedition 370. There, we measured the vitrinite reflectance which is an index of the maximum temperature experienced by the sediments. Comparing the measured reflectance and the model values calculated by assuming the past heat flow, it was found that Site C0023 experienced a higher heat flow than the present, which was approximately 160 mW/m². However, the vitrinite reflectance is significantly higher than that in the above model just below the décollement, which suggested that another thermal anomaly originated directly under the décollement in addition to the high heat flow from the basement. With assumptions on the temperature of the heat source and the duration of heating below the décollement, we calculated the vitrinite reflectance in different models.

As a result, it was found that heat source temperature of 200˚C and heat generation duration of 500-1000 years are required just below the décollement to explain the depth distribution of the measured values. At Site C0023, a high pore pressure zone is distributed just below the décollement, which can serve as a path for fluids from deeper part. Considering that the temperature at the depth corresponding to the seismogenic zone in the Muroto area of the Nankai Trough is approximately 200˚C, and that a specific high temperature has not been confirmed just below the décollement of C0023 at present, the origin of the high-temperature fluid would be the deep seismogenic zone. Furthermore, the advection of high-temperature fluids is thought to be intermittent. In other words, the high reflectance just below the décollement is considered to indicate the advection of the high-temperature fluid from deep to shallow areas at the time of past earthquakes.